Full duplex radio frequency (RF) transmitter-receivers and transceivers have been capable of simultaneously transmitting and receiving RF signals. Typically, the transmit bands and the receive bands have been offset from each other to minimize interference at the receiver from the outgoing transmitter signals. Even though the transmit bands are different from the receive bands, components from the transmitted signal may still leak into the received signals at the receiver causing interference. This interference has reduced the sensitivity of the receiver so that the receiver has a reduced capability of receiving fainter signals.
The components leaking into the received signal may be caused by the nonlinear properties of the transmitter-receiver circuits. For example, while transmitter amplifiers have been designed to linearly amplify outgoing signals, in practice manufacturing and other limitations have resulted in nonlinear amplification of different components of the outgoing signal. This nonlinear amplification has distorted the incoming signals received at the receiver. In other instances, other nonlinear properties of the transmitter-receiver circuits, such as nonlinear approximation errors during signal sampling have also distorted the received signals.
Existing solutions for minimizing distortion at the receiver have focused on improving the linearity of these circuits and reducing nonlinear sources of distortion by including additional filters to remove nonlinear terms and improve filter adaptation and convergence rates. These improvements and additional filters have resulted in larger, more expensive circuits with additional elements consuming more power. The larger, more expensive, and power consuming circuits are not suitable for smaller sized portable devices designed to consume less power, such as smart phones, tablets, and other mobile RF devices.
These existing solutions are also not suitable in systems including multiple transmitters and one or more receivers. In systems with multiple transmitters, the signals at each receiver may be distorted by outgoing signals from more than just one transmitter. Existing circuits for minimizing distortion in multiple transmitter systems are too large, expensive, and power consuming for these smaller, portable, battery powered devices.
Finally, transmitters and receivers in these smaller, portable devices may only use a subset of available channels in a designated signal band. In instances where these devices only use a subset of the available channels, any distortion in the unused channels of the signal band need not be removed from the incoming received channels. Existing solutions focused on removing distortions from the entire signal band, including in unused channels. Accordingly, the solutions required extra processing time and power to remove distortions in unused channels even though there was no need to do so.
There is a need for a smaller, low power transmitter noise correction circuit that is able to cost efficiently minimize transmitter signal distortion in received signals. There is also a need for a smaller, low power transmitter noise correction circuit suitable for portable devices that is able to remove distortions from only active channels and/or from multiple transmitters.